Spark plug having fusion zone

Abstract

A spark plug ( 1 ) includes a center electrode ( 5 ), an insulator ( 2 ), a metallic shell ( 3 ), a ground electrode ( 27 ), and a noble metal tip ( 32 ) provided on at least one object member of the center electrode and the ground electrode. One end surface of the noble metal tip is joined to the object member via a fusion zone ( 35 ). The fusion zone includes a first fusion zone ( 351 ) formed through radiation of a laser beam or the like to the boundary between the object member and the one end surface of the noble metal tip along a perimetrical direction of the noble metal tip, and a second fusion zone ( 352 ) formed through radiation of the laser beam or the like from the side from which the laser beam or the like has been radiated in forming the first fusion zone, and intersecting with the first fusion zone.

The invention claimed is: 
     
       1. A spark plug comprising:
 a rod like center electrode extending in a direction of an axis; 
 a tubular insulator provided around the center electrode; 
 a tubular metallic shell provided around the insulator; 
 a ground electrode whose proximal end is welded to the metallic shell and whose distal end faces the center electrode; and 
 a columnar noble metal tip formed from a noble metal alloy and provided on at least one object member of the center electrode and the ground electrode; 
 one end surface of the noble metal tip being joined to the object member via a fusion zone formed through radiation of a laser beam or an electron beam from a side toward a side surface of the noble metal tip; 
 the spark plug being characterized in that the fusion zone comprises: 
 a first fusion zone formed through radiation of the laser beam or the electron beam along the entire circumference of a boundary between the object member and the one end surface of the noble metal tip along a perimetrical direction of the noble metal tip, and 
 a second fusion zone formed through radiation of the laser beam or the electron beam from the side from which the laser beam or the electron beam has been radiated in forming the first fusion zone, and intersecting with the first fusion zone; wherein 
 the thickness of the second fusion zone along a longitudinal center axis of the noble metal tip is greater than the thickness of the first fusion zone along the longitudinal center axis of the noble metal tip. 
 
     
     
       2. The spark plug according to  claim 1 , wherein the noble metal tip is joined to at least an inner side surface of the ground electrode, and the fusion zone is formed through radiation of the laser beam or the electron beam from a side toward at least one of a distal end surface and opposite side surfaces of the ground electrode, and
 when the noble metal tip and the fusion zone are viewed from the side from which the laser beam or the electron beam has been radiated to the surface of the ground electrode, 
 assuming that a portion of the fusion zone located between the ground electrode and the noble metal tip is equally divided into three segmental regions along a width direction of the noble metal tip, the first fusion zone and the second fusion zone are in contact with each other in at least a center one of the three segmental regions. 
 
     
     
       3. The spark plug according to  claim 1 , wherein
 the noble metal tip is joined to at least the ground electrode, and the fusion zone is formed through radiation of the laser beam or the electron beam from a side toward at least one of a distal end surface and opposite side surfaces of the ground electrode, and 
 when the noble metal tip and the fusion zone are viewed from the side from which the laser beam or the electron beam has been radiated to the surface of the ground electrode, 
 assuming that a portion of the fusion zone located between the ground electrode and the noble metal tip is equally divided into three segmental regions along a width direction of the noble metal tip, the first fusion zone and the second fusion zone are in contact with each other in at least opposite end ones of the three segmental regions. 
 
     
     
       4. The spark plug according to  claim 1 , wherein
 the noble metal tip is joined to at least the ground electrode, and 
 through radiation of the laser beam or the electron beam from a side toward each of a distal end surface and opposite side surfaces of the ground electrode, the second fusion zone is formed on each of the distal end surface and the opposite side surfaces of the ground electrode. 
 
     
     
       5. The spark plug according to  claim 1 , wherein
 the noble metal tip is joined to at least the ground electrode; 
 a plurality of the second fusion zones are formed; and 
 as viewed from a side toward the other end surface of the noble metal tip, the second fusion zones are formed at positions located symmetrically with respect to a center axis of the noble metal tip. 
 
     
     
       6. The spark plug according to  claim 1 , wherein
 the noble metal tip is joined to at least the ground electrode; 
 a plurality of the second fusion zones are formed; and 
 as viewed from a side toward the other end surface of the noble metal tip, the second fusion zones are formed at positions located symmetrically with respect to a straight line which extends along a longitudinal direction of the ground electrode and passes through a center axis of the noble metal tip. 
 
     
     
       7. The spark plug according to  claim 1 , wherein
 the noble metal tip is joined to at least the ground electrode; 
 a plurality of the second fusion zones are formed; and 
 as viewed from a side toward the other end surface of the noble metal tip, the second fusion zones are formed at positions located symmetrically with respect to a straight line which extends along a direction orthogonal to a longitudinal direction of the ground electrode and passes through a center axis of the noble metal tip. 
 
     
     
       8. The spark plug according to  claim 1 , wherein
 the noble metal tip is joined to at least the center electrode; 
 the first fusion zone is formed along the entire circumference of the noble metal tip; 
 a plurality of the second fusion zones are formed; and 
 as viewed from a side toward the other end surface of the noble metal tip, the second fusion zones are formed at positions located symmetrically with respect to a center axis of the noble metal tip. 
 
     
     
       9. The spark plug according to  claim 1 , wherein the first fusion zone has a maximum thickness of 0.3 mm or less along a center axis of the noble metal tip. 
     
     
       10. The spark plug according to  claim 1 , wherein a length of an outer surface of the second fusion zone along a perimetrical direction of the noble metal tip is 30% or more of a length of an outer surface of the first fusion zone along the perimetrical direction of the noble metal tip. 
     
     
       11. The spark plug according to  claim 1 , wherein a length of an outer surface of the second fusion zone along a perimetrical direction of the noble metal tip is 50% or more of a length of an outer surface of the first fusion zone along the perimetrical direction of the noble metal tip. 
     
     
       12. The spark plug according to  claim 1 , wherein a length of an outer surface of the second fusion zone along a perimetrical direction of the noble metal tip is 70% or more of a length of an outer surface of the first fusion zone along the perimetrical direction of the noble metal tip. 
     
     
       13. The spark plug according to  claim 1 , wherein
 as viewed on a plane of projection which is orthogonal to a center axis of the noble metal tip and on which the noble metal tip and the fusion zone are projected along the center axis, 
 a projected overlap region of the noble metal tip and the fusion zone accounts for 50% or more of a projected region of the noble metal tip. 
 
     
     
       14. The spark plug according to  claim 8 , wherein assuming that an outer circumferential surface of the fusion zone is equally divided into three segmental regions along a circumferential direction thereof, the second fusion zone exists in each of the three segmental regions. 
     
     
       15. A spark plug comprising:
 a rod like center electrode extending in a direction of an axis; 
 a tubular insulator provided around the center electrode; 
 a tubular metallic shell provided around the insulator; 
 a ground electrode whose proximal end is welded to the metallic shell and whose distal end faces the center electrode; and 
 a columnar noble metal tip formed from a noble metal alloy and provided on at least one object member of the center electrode and the ground electrode; 
 the spark plug being characterized in that: 
 one end surface of the noble metal tip is joined to the object member via a fusion zone which, as viewed along the axial direction from the side from which the laser beam or the electron beam has been radiated, has a wave shape formed by radiating a laser beam or an electron beam from a side toward a side surface of the noble metal tip in such a manner as to intersect with a boundary between the noble metal tip and the object member, 
 wherein the wave shape has opposing crests formed on opposite sides of the boundary between the noble metal tip and the object member, 
 the wave shape is substantially sinusoidal, and 
 the fusion zone comprises a plurality of segmental fusion zones formed across the boundary between the object member and the one end surface of the noble metal tip. 
 
     
     
       16. The spark plug according to  claim 15 , wherein the noble metal tip is joined to at least an inner side surface of the ground electrode, and the fusion zone is formed through radiation of the laser beam or the electron beam from a side toward at least one of a distal end surface and opposite side surfaces of the ground electrode, and
 as viewed from the side from which the laser beam or the electron beam has been radiated, a portion of an outer surface of the fusion zone located on a boundary between the noble metal tip and the ground electrode has a length which is 30% or more of a length of the boundary. 
 
     
     
       17. The spark plug according to  claim 15 , wherein the noble metal tip is joined to at least an inner side surface of the ground electrode, and the fusion zone is formed through radiation of the laser beam or the electron beam from a side toward at least one of a distal end surface and opposite side surfaces of the ground electrode, and
 as viewed from the side from which the laser beam or the electron beam has been radiated, a portion of an outer surface of the fusion zone located on a boundary between the noble metal tip and the ground electrode has a length which is 50% or more of a length of the boundary. 
 
     
     
       18. The spark plug according to  claim 15 , wherein the noble metal tip is joined to at least the ground electrode, and
 through radiation of the laser beam or the electron beam from a side toward each of a distal end surface and opposite side surfaces of the ground electrode, the segmental fusion zones are formed on the distal end surface and the opposite side surfaces of the ground electrode. 
 
     
     
       19. The spark plug according to  claim 15 , wherein
 the noble metal tip is joined to at least the ground electrode, and 
 as viewed from a side toward the other end surface of the noble metal tip, the segmental fusion zones are formed at positions located symmetrically with respect to a center axis of the noble metal tip. 
 
     
     
       20. The spark plug according to  claim 15 , wherein
 the noble metal tip is joined to at least the ground electrode, and 
 as viewed from a side toward the other end surface of the noble metal tip, the segmental fusion zones are formed at positions located symmetrically with respect to a straight line which extends along a longitudinal direction of the ground electrode and passes through a center axis of the noble metal tip. 
 
     
     
       21. The spark plug according to  claim 15 , wherein
 the noble metal tip is joined to at least the ground electrode, and 
 as viewed from a side toward the other end surface of the noble metal tip, the segmental fusion zones are formed at positions located symmetrically with respect to a straight line which extends along a direction orthogonal to a longitudinal direction of the ground electrode and passes through a center axis of the noble metal tip. 
 
     
     
       22. The spark plug according to  claim 15 , wherein
 the noble metal tip is joined to at least the center electrode, and 
 a portion of an outer surface of the fusion zone located on a boundary between the noble metal tip and the center electrode has a length which is 30% or more of a length of the boundary. 
 
     
     
       23. The spark plug according to  claim 15 , wherein
 the noble metal tip is joined to at least the center electrode, and 
 a portion of an outer surface of the fusion zone located on a boundary between the noble metal tip and the center electrode has a length which is 50% or more of a length of the boundary.